1. Field of the Invention
This invention relates to D/A converters used in systems where it is important to be able to controllably alter the full-scale output of the converter, for example, where the converter is used to develop signals for various types of CRT displays.
2. Background of the Invention
DACs are extensively used for developing analog signals for a wide variety of functions. One important application is that of controlling the electron beam intensity of CRT color guns. In such application, it frequently is desirable to change the full-scale output signal level of the DAC. Generally this has been done by altering the value of some external component such as a current-setting resistor (RSET), or changing the resistance of an output load resistor of a current output DAC, or altering the output of a VREF or IREF reference source used with the DAC. Such means are satisfactory where the DAC is set up permanently at the factory, but not where there is need to make changes from time to time in the field, such as when used with instrument actuators of different input ranges, or to supply CRT video inputs with different Full Scale video levels.
No optimum DAC design arrangement exists for making full-scale output changes on a real time basis, such as might be needed in a video DAC or RAMDAC. A variety of expedients have been adopted, such as the use of a variable resistor network in place of the current setting resistor, or the use of a programmable gain amplifier on the DAC output. However, none of these has been fully satisfactory, due to lack of accuracy, matching, frequency response, or other limitations. Thus there exists a need for developing a DAC design capable of having its full-scale output level altered quickly and easily.
A CRT video DAC with adjustable setting of full-scale output must meet special requirements applicable to CRT displays. This is because the video-DAC in a graphics system typically generates only the Black-White portion of a graphics video waveform. The other portions (pedestal, sync level) are usually generated by auxiliary current sources in the DAC. When the full scale amplitude of the video signal is adjusted, these other levels must also vary by the same amount in order to keep correct ratios. This is also true for the video signal in a High-Definition TV system (HDTV) which contains an extra positive sync level which must also track. The extra Positive Sync Tip often is developed by an additional current source, external to the DAC, and driven by another sync signal (herein referred to as a Trisync signal). Such an arrangement however presents difficulties in achieving effective coordination of the timing signals, resulting for example in the positive and negative portions of the sync signal "moving about" with respect to each other, and having different amplitudes and positions.
Also of importance is the fact that if one alters the full-scale output signal of the above-described type of prior art video DAC, it is difficult to make a corresponding change in the output of the external current source. Thus there is significant likelihood that there will be incorrect matching of positive and negative synchronizing signals that should track in order to achieve proper operation of the display system.